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Summary All IB Biology Experimental Case Studies
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A summary of the details and findings of all the experimental case studies and scientists required for the entire IB Biology course (from Topic 1 to 11)
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Biology Required Experiments and ScientistsMembrane models
Davison-Danielli
Analysis of evidence from electron microscopy that led to the proposal of the Davson-Danielli model
- Membranes partially permeable, organic compounds penetrate faster than H2O → nonpolar regions
- Membranes composed of mainly lipids and proteins, amphipathic properties of phospholipids make it so that
when shaken with water they form micelles
- Sandwich Model - trilaminar, a phospholipid bilayer between two protein layers
- Evidence for model - electron microscopy showed two dark lines separated by a lighter band (proteins appear
darker than phospholipids)
Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model
- Evidence that disproved model
- Freeze-fracture splitting found irregular rough membrane surfaces - proteins embedded in the middle
- Protein analysis found that some proteins were insoluble (hydrophobic surfaces) and varied in size
- Fluorescent-tagging of membrane proteins were quickly mixed - proteins could move
- Result - Singer-Nicolson model later proposed
Singer-Nicolson
Drawing of the fluid-mosaic model
- Fluid-mosaic model - a stable but fluid phospholipid bilayer with embedded proteins
- Possible due to the amphipathic properties of phospholipids
- Shows glyco, peripheral and integral proteins
Cell Theory
Pasteur
Evidence from Pasteur’s experiments that spontaneous generation of cells and organisms does not now occur on Earth
- Experiment
- Added boiled chicken broth into two sterilised (open-ended and swan-necked) flasks
- After some time at room temperature, the broth in the open flask is discoloured and cloudy, while the
broth in the curved-neck flask did not change
- When the swan-neck was snapped, exposing the broth to air, organisms soon appeared (turned cloudy)
- Result - proved that spontaneous generation cannot occur now → cells must come from preexisting cells
Miller-Urey
The first cells must have arisen from non-living material
- Experiment - recreated the postulated conditions of prebiotic Earth using a closed system
- Simulated: volcanic activity - water boiled to vapour; atmosphere - vapour mixed with a variety of gases
(H2, CH4, NH3) to create a reducing atmosphere (no oxygen); lightening - exposed to an electrical
discharge (as an energy source and catalyst for reactions)
- Mixture then allowed to cool (concentrating components) and left for a period of ~1 week
- Analysis of condensed mixture found traces of simple organic molecules (amino acids and hydrocarbons)
- Result - showed how abiogenesis was possible → the first cells must have arisen from non-living material
DNA
Crick and Watson
Crick and Watson’s elucidation of the structure of DNA using model making
- Correctly proposed the structural organisation of the DNA molecule
- Ethical issue - used Rosalind Franklin’s data from x-ray crystallography without her knowledge or consent
- Result - DNA structure → double helix, antiparallel strands, composed of nucleotides and complementary bases
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